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What Is an Exoplanet?

Sixty three light-years away in a constellation called Vulpecula lies a planet called HD 189733b. While its name may sound unremarkable, astronomers proved this week that HD 189733b is, indeed, special: It's deep-blue in color and covered in clouds of liquid glass.

HD 189733b is an exoplanet. Located outside of our solar system, it's just one of more than 900 similar planets we've identified. However, scientists estimate there are at least100 billion exoplanets — and that's just in the Milky Way Galaxy alone.

The science world is buzzing this week about HD 189733b because, for the first time ever, we were able to identify an exoplanet's color. However, HD 189733b's characteristics coincide with what we already know about these types of planets: It's similar to Jupiter and orbits a host star.

The latter is especially important because scientists base their exoplanet estimate on the fact that they orbit stars.

"It's a staggering number, if you think about it," said Jonathan Swift, a postdoctoral student at Caltech and lead author of a January 2013 paper estimating the number of exoplanets in our galaxy. "Basically, there's one of these planets per star."

Those host stars are a tremendous help for scientists looking to identify properties of exoplanets. For example, astronomers used HD 189733b's star to measure its color using the Hubble Space Telescope's light-splitting spectrograph.

Most of the exoplanets scientists have spied are similar to Jupiter and Neptune, but that may just be because larger objects are easier to see from great distances. We've also identified Earth-sized planets beyond our solar system. Kepler, a spacecraft NASA launched in search of Earth-like planets, first found two Earth-sized planets orbiting a sun-like star in 2011.

But the search for alien life has always intrigued humans, and it's no different when it comes to exoplanet study. The ultimate goal is to positively identify a habitable planet that could host life. More than likely, there are billions of Earth-like planets. The key, however, is finding those planets in a habitable zone, meaning that it's close enough to a star to have liquid, but far enough away to be cool enough to support life.

Scientists have already identified many zones that fit that criteria. Most recently, astronomers found three potentially habitable planets 22 light-years away, around the star Gliese 667C. The team believes there are at least six planets orbiting that one star.

That's only a fraction of what awaits our discovery. Fewer than 5% of known exoplanets can be seen with a telescope, and ground-based missions often only spot those very large exoplanet systems.

NASA's Kepler has helped open a new frontier, discovering more than 2,700 potential alien worlds. However, it suffered a major malfunction in May that could cripple the spacecraft. Nevertheless, Kepler delivered mountains of data we'll pore over for years.

Future space-based missions — such as European Space Agency's Gaia spacecraft set to launch in October — may give us a better look at these planets.

Famous Exoplanets of the Milky Way

Alpha Centauri Bb

Alpha CentauriBb is located in the star system closes to Earth, located just 4.37 light-years away. However, a June, 2013 report questions the existence of the planet, fueling a debate between astronomers.

Image courtesy of NASA

Kepler-10b

Discovered in 2011, Kepler-10b is the smallest-known exoplanet, measuring 1.4 times the size of Earth.

Image courtesy of NASA

Gliese581 g

Twenty light-years away lies Gliese581 g, a rocky planet orbiting in a habitable zone. If its existence is confirmed, it will be the most Earth-like planet we've found to date.

Image courtesy of NASA

Kepler-22b

Kepler-22b is the first super-Earth located in the habitable zone of a Sun-like star. Scientists do not yet know if the planet has a predominantly rocky, gaseous, or liquid composition.

Image courtesy of NASA

Kepler-62f

Located 1,200 light-years from Earth, Kepler-62f is a super-Earth-size planet in the habitable zone of a star smaller and cooler than the sun.

Image courtesy of NASA

Gliese 667C's Planets

In June, a team of scientists discovered three potentially habitable planets around the a star called Gliese 667C, which is 22 light-years from Earth.

Image courtesy of ESA

Surprise Exoplanet HD 95086 b

Astronomers snapped a photo in June of a giant gaseous planet located 300 light-years away from Earth. The planet is four to five times bigger than Jupiter.

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